In the employment of turbo-chargers or other supercharging devices (for example, gas-dynamic pressure wave machines) for compression of the air conveyed to an engine, in general a determined maximum charging pressure must not be exceeded. In order to avoid a pressure excess, often a so-called "waste gate" is employed. A waste gate is a valve through which high pressure exhaust gas is blown off as soon as the maximum permissible charging pressure is reached. In a pressure wave machine (for the functioning of which reference is made to Swiss Pat. No. 378,595 or to the publication CH-T 123,143 of the assignee) the employment of a waste gate is not totally free of problems. For instance, if too much exhaust gas is blown off, the exhaust gas still carried to the charger is no longer sufficient to ensure a complete scavenging in the low pressure scavenging period. In this case a part of the exhaust gas is again compressed, and of this a part again arrives in the charging air channel. This has the consequence that the charging air temperature is undesirably high and that the intake capacity of the engine is impaired in the range of higher revolutions.